Pipes and pipelines in general normally require cleaning, testing or gauging, and for this purpose it is well known to use a so-called “pig.” The pig is designed to fit closely within the pipe and is caused to travel along the pipe by admitting fluid under pressure behind the pig. Pigs are also used in operation of a pipeline to separate different fluids (liquids and gases) delivered in succession. The pigs are of various designs, the more common type being of spool shape with annular sealing members around the two flanges of the spool. Other pigs are of generally cylindrical shape, formed of resilient material such as foamed plastics, and it is also common practice to use spherical pigs, either of a solid resilient material, or inflated or inflatable.
Pipelines that are used to transport products such as petroleum, gas or other fluids can become blocked or inefficient through the build up of deposits on the pipe walls. The deposits can be foreign material, detritus, or natural waste products such as, for example, paraffin, calcium, wax and hydrates. It is well know to insert a pig into the pipe in order to clean it. The pig is transported by the fluid pressure along the pipe and has an outer periphery that is of a size that is similar to the diameter of the inside surface of the pipe. Thus, as the pig travels along the pipe—along with fluid flow in the pipe—it serves to remove deposits from the inner surface by scraping or brushing, or simply by pushing the deposits ahead of it as it travels to a point where it can be removed along with the released deposits. Such mono-directional pigs, which are transported along with the fluid flow, may become stuck when it encounters large amounts of pipe wall deposits, and thus form a permanent plug in the pipeline.
In the oil and gas industry, the necessity of pigging operations is especially significant. Severe problems often occur when hydrocarbon fluids are transported in long subsea pipelines at large depths and in cold waters. Such problems may include the formation of obstructions in the pipeline, in the form of hydrates or other deposits such as ice, wax and debris (e.g. asphaltenes, sand). The initially warm well fluid is cooled down by cold seawater, thereby inducing condensation, precipitation and hydrate and wax formation/crystallization. A number of methods of removing such wax and hydrate formation, or preventing the formation of such, exist:                Adding chemicals (such as methanol or mono-ethylene glycol; MEG) to the well fluids. This is a costly method (installation, self-cost and regeneration plants) and is detrimental to the environment.        Using direct electric heating (DEH), i.e. arranging electrical cables along the pipeline in order to maintain the well fluids at a temperature above the temperature at which wax precipitates (“wax appearance temperature”—WAT). This method entails costly equipment, installation work and operation. Power availability and infrastructure to transfer it, is a major cost driver when producing far from land or topside installation        Thermal insulation in the form of applying thermal cladding (insulation) around the pipeline and/or burying it in the seabed. Alternatively a pipe-in-pipe configuration. Both require additional materials and increase the cost of pipe fabrication and installation.        Rock dumping and dredging pipelines is done mainly to insulate the pipes further, keeping the flow warm. This is a time consuming activity that also represent extra costs.        Using a pig, as described above. There are several disadvantages associated with the known pigs. A pigging system typically comprises a pig launching station and a retrieving station which each comprise an assembly of isolation valves, a trap barrel, an entry hatch and a bypass valve that enable an operator to launch a pig into the pipeline safely and to retrieve it at the other end. The trap barrels are generally closed at one end and situated outside the main pipeline. The system tends take up a large volume and is heavy. Also, the well stream production must in many cases be reduced in order not to impose too high a pressure on the pig.        All the measures taken to prevent formation or hydrate and wax deposits today have limits when it comes to transportation distance. The longer the pipe, the higher the cost. For long step-out fields like the Stockman, present methods are not technically or economically applicable.        
A simple and reliable system for ensuring subsea transport of hydrocarbons over long distances is to allow so-called “cold flow”. If the well stream fluids, pipeline wall and the ambient seawater all are at the same temperature, wax deposits do not form on the interior pipe wall surface, but are transported together with the well fluid without problems. Cold flow is normally achieved by allowing the well stream to be cooled to ambient seawater temperature simply by heat exchange through the pipeline wall. However, severe hydrate and wax formation will take place in the pipeline section where cooling takes place. This relatively short cooling section will therefore have to be pigged more frequently.
The state of the art includes WO 2006/068929 A1 which describes a system for assuring subsea hydrocarbon production flow in pipelines. A hydrocarbon production flow is chilled in a heat exchanger, whereby solids form, and a pig is used for periodically removing deposits and placing them in a slurry. A closed loop pig launching and receiving system is disclosed. A production flow from wells is transported from a manifold to a cold flow module through flow line. The cold flow module is connected to a chilling loop/heat exchanger, which returns to cold flow module. Pig launcher and handling systems are connected to the heat exchanger. The pig is driven by the fluid flow and may alternatively be launched through the heat exchanger and recovered at a terminus, whether that is on an offshore platform or onshore.
The state of the art also includes WO 02/42601, describing an alternative pig propulsion method.
The present applicant has devised and embodied this invention to overcome shortcomings of the prior art and to obtain further advantages.